Glass comprising a porous anti-reflection surface coating and method for producing one such glass

ABSTRACT

A coated glass product includes a glass substrate and a porous antireflection surface coating including SiO 2 -based particles having a first particle fraction including a first characteristic particle size range and a second particle fraction having a second characteristic particle size range that is different from the first characteristic particle size range. In addition, a method for producing the coated glass product is provided.

[0001] The invention relates to a glass provided with a porousantireflection surface coating on the basis of SiO₂ particles. It alsorelates to a method for the production of such a glass as well as to theuse of such a glass.

[0002] When light passes through the interface of two media havingdifferent refractive indices, part of the radiation is reflected. Forinstance, when light falls perpendicularly upon a glass pane, thereflected fraction of incident light is about 4% due to the differencebetween its refractive index of n=1.5 and the refractive index of air ofn=1. The same fraction of about 4% is also reflected when the lightexits from the glass. Thus, at the maximum, an amount of 92% of theincident light passes through a conventional glass pane, which can causean undesired loss of efficiency, especially when a glass pane isemployed to cover solar collectors or other optically sensitiveelements. For this reason, precisely when it comes to the covering ofsolar collectors, it is desirable to use so-called antireflection-coatedglass in which the radiant transmittance of the glass in question isenhanced by means of a coating on the surfaces.

[0003] In order to render glass anti-reflective, multiple layers can beapplied onto the surface. In this process, on the basis of theinterference principle, alternating layers having high and lowrefractive indices are applied. Owing to interferences of the partialwaves that are reflected on the appertaining interfaces between thematerials having different refractive indices, these partial waves areextinguished within a certain wavelength range, so that a particularlyhigh radiant transmittance can be achieved for these wavelengths. Suchalternating layer systems, however, are wavelength-selective and thusnot suitable for use in a broadband spectrum. As a result, such coatedglass is not suited for covering, for example, solar collectors, whereit is crucial to achieve the best possible passage of light within theentire solar spectrum.

[0004] An alternative for rendering glass anti-reflective consists ofapplying a single layer onto the glass surface in question. Here, forphysical reasons, an especially high transmission can be achieved if thesurface layer has a refractive index equal to the square root of therefractive index of glass, in other words, a refractive index of about1.22. In this case, the reflection of light having a wavelength that isfour times the layer thickness is virtually zero, so that light havingthis wavelength is transmitted completely. Owing to the comparativelyflat functional wavelength-dependence of the radiant transmittance,however, the latter is still particularly high for wavelengths thatdiverge from this. For this reason, precisely in the case of glass forcovering solar collectors or other optically sensitive elements, effortsare aimed at obtaining a coating with a material that has a refractiveindex that is as close to 1.22 as possible.

[0005] Such a surface coating for glass can be produced by selectivelyetching the glass. For instance, etching soda-lime glass, for example,with hydrofluoric acid or hexafluorosilicic acid, can yield surfacelayers having a refractive index of around 1.27 which already comes veryclose to the desired result. The surface layers produced in this manner,in addition to good optical properties, also have relatively goodmechanical properties, especially a high mechanical resistance toabrasion. Therefore, glass produced in this way is also fairlywell-suited for everyday use. This production method has the drawback,however, that it calls for the use of acids that are extremely harmfulto the environment and aggressive, which then requires correspondinglycomplex disposal measures and commensurate precautions for handling suchmaterials.

[0006] As an alternative, the glass can also be coated by means of anadditive application of coating material. On the one hand, highrequirements have to be produced of coated glass made in this manner interms of its optical properties, particularly with respect to arelatively small refractive index that is as close to 1.22 as possible.On the other hand, high requirements also have to be made of themechanical properties of the coating of these types of glass, especiallytheir abrasion resistance, in order to render them suitable for everydayuse, even under relatively harsh conditions. With an eye towards theserequirements, antireflection surface coatings on the basis of SiO₂particles have proven to be particularly well-suited.

[0007] In order to attain a suitably low refractive index of the surfacelayer that comes as close to n=1.22 as possible, the antireflectionsurface coatings on the basis of SiO₂ particles are normally poroussince an acceptably lower refractive index can already be achieved bymerely thinning the coating material with air. Such porousantireflection surface coatings on the basis of SiO₂ particles arenormally characterized by more or less loose SiO₂ particles joinedtogether and having an essentially uniform particle size.

[0008] When glass is coated with such a porous antireflective surfacecoating on the basis of SiO₂ particles, this is normally done usingso-called sols in which [SiO_(X)(OH)_(Y)]_(n) particles are mixed withsolvents and optionally with a stabilizer. On the basis of such sols,coating solutions can be prepared into which the glass to be coated canbe dipped, as a result of which the layer-forming sol precipitates ontothe surface of the glass.

[0009] German patent DE 199 18 811 A1 discloses the use of such a sol onthe basis of an alcohol-water mixture for the production of a porousantireflection surface coating on the basis of SiO₂ particles. Theantireflection surface coating produced here exhibits relatively goodoptical properties and is also sintering-stable so that the opticalproperties of an antireflection surface coating applied in this mannerdoes not deteriorate to any appreciable extent, even during a subsequentthermal treatment of the coated glass, for example, in order to producethermally toughened safety glass. However, for this coating, it has beenfound that the abrasion resistance does not meet the requirements forlong-term use. For example, in the case of glass with such a porousantireflection surface coating, the test of the abrasion resistanceaccording to DIN EN 1096-2 by means of the crockmeter test shows thatmarked damage to the layer already occurs after ten cycles and severedamage occurs after 100 cycles.

[0010] As an alternative, a porous antireflection surface coating onglass can also be produced using sols on the basis of aqueous systemsthat contain less than 1% organic components. The surface layers thatcan be produced by using such sols, which contain surfactants and whichare essentially purely aqueous, increase the solar transmission of alow-iron soda-lime glass that is coated with such surface layers to asmuch as 95.3%, whereby the antireflection surface coating has arefractive index of 1.29. As it turned out, an antireflection surfacecoating produced in this manner is mechanically very stable and abrasionresistant, whereby the test of the abrasion resistance ascertained bymeans of the crockmeter test according to DIN EN 1096-2 revealed onlyslight changes to the layers even after 100 cycles. However, a drawbackof antireflection surface coatings produced in this manner is that, dueto the manufacturing process, the layers can exhibit inhomogeneities.Particularly in terms of visual appearance, crosswise streaking occursthat can be ascribed to periodical differences in the layer thicknesswithin the range of a few nanometers. Such streaking can be detrimental.Moreover, the antireflection surface coatings that can be produced byusing such an aqueous sol only yield unsatisfactory optical results whenused for coating prism-cut glass, whereby the achievable radianttransmittance is only about 93.6%.

[0011] Consequently, none of the above-mentioned antireflection surfacecoatings fully meet the stipulated requirements for a coating that isdurable and that can be used for covering glass for solar collectors orother optically sensitive elements.

[0012] Consequently, the invention is based on the objective ofproviding a glass that has a porous antireflection surface coating onthe basis of SiO₂ particles and that, on the one hand, has especiallygood optical properties in terms of a high radiant transmittance oflight within the entire solar spectrum and, on the other hand, has aparticularly high mechanical strength, especially a particularly highmechanical abrasion resistance. Moreover, a method for the production ofsuch a glass as well as an especially advantageous use of the glass areto be proposed.

[0013] Regarding the glass, this objective is achieved according to theinvention in that the antireflection surface coating on the basis ofSiO₂ particles comprises at least two particle fractions that differfrom each other in terms of their characteristic particle size.

[0014] In particular, the surface coating contains relatively largeparticles on the one hand, and relatively small particles on the otherhand.

[0015] The invention is based on the consideration that theantireflection surface coating should be configured so as to beespecially flexible in its structural composition because of thedifferent requirements, namely, the good optical properties on the onehand and the high abrasion resistance on the other hand. In thiscontext, the structural components or sub-components of theantireflection surface coating provided should be such that each one canbe specifically optimized in order to meet one of the above-mentionedrequirements. As was surprisingly found, the particle size of the SiO₂particles is a suitable parameter for distinguishes between thesedifferent components that can each be optimized for a particularrequirement.

[0016] In particular, relatively small SiO₂ particles have an especiallyhigh surface reactivity. Consequently, the SiO₂ particles with arelatively smaller particle size tend towards aggregation oragglomeration, which especially allows the formation of a layer ofuniform thickness, especially in terms of any possible streak formation.Before this agglomeration occurs, the relatively small particles can bemade available to the relatively large particles for purposes ofreaction. In this manner, the surface of the relatively large SiO₂particles is modified in such a way that they, too, tend to form layershaving an especially homogeneous layer thickness. Such SiO₂ particleshaving relatively large dimensions, which can especially be present inthe form of similar round beads or “monospheres”, contribute to a greatextent to the overall stability of the system, especially to theskeleton stability and to the adhesion of the surface layer to theunderlying glass. It is precisely this combination of these SiO₂particles that are kept relatively large with the SiO₂ particles thatare kept relatively small that practically avoids a deterioration of theoptical properties through the use of the SiO₂ particles that are keptrelatively large.

[0017] The at least two particle fractions in the form of a binary orbimodal system having different characteristic particle sizes account,for example, for a particle size distribution in which the SiO₂particles that make up the antireflection surface coating makeespecially significant contributions to at least two size ranges.Therefore, in this context, two particle fractions with differentcharacteristic particle sizes are present when, for instance, theparticle size distribution in two particle size intervals assumesrecognizable large values, when the surface area integral under theparticle size distribution is relatively large and/or when relativemaxima occur in the particle size distribution. The characteristicparticle size of each particle fraction can then be defined, forinstance, by the maximum point in each particle size interval, by themean value of the particle size distribution in each particle sizeinterval or else by the mean value of the particle sizes in eachparticle size interval, whereby the particle size of the particles ofeach particle fraction can assume a certain distribution or bandwidtharound each characteristic particle size.

[0018] As was surprisingly found, especially high-quality opticalproperties and especially a very homogeneous layer thickness withvirtually no streaking can be achieved if the particles of therelatively small SiO₂ particle fraction have particle sizes amounting toa few nm. Consequently, the surface coating advantageously has a firstparticle fraction with particle sizes within the range from 4 nm to 15nm.

[0019] As the second fraction, it is advantageous to have SiO₂ particleswith a mean particle size of about 20 nm to 60 nm. Consequently, thesurface coating in an alternative or additional advantageous embodimenthas a second particle fraction with a mean particle size of 20 nm to 60nm, whereby the standard deviation of the particle size distribution ofthis particle fraction is preferably 20% at the maximum.

[0020] Corresponding to the functional allocation of the particlefractions, relatively many SiO₂ particles having small dimensions areadvantageously combined with relatively few SiO₂ particles having largerdimensions. In an especially advantageous embodiment, the surfacecoating has a ratio of the number of particles of the first fraction tothe number of particles of the second fraction of 3000:1 to 100:1,preferably 1000:1 to 250:1.

[0021] In an advantageous embodiment, the coated class is configured asso-called toughened safety glass. Safety glass—in addition to havinggreater breaking strength—is characterized in that, if the glass breaks,it does not disintegrate into relatively large sharp-edged shards, butrather into a large number of relatively small, dull-edged fragments.Glass that is configured as such safety glass can be obtained byso-called thermal toughening, whereby the glass is first heated totemperatures of at least 600° C. [1112° F.] and subsequently thermallyquenched, for example, by blowing air against it. The actual tougheningprocess can be carried out with conventional toughening methods. Inparticular, the so-called vertical toughening technique, the so-calledhorizontal toughening technique in a continuous process, or theso-called horizontal toughening technique in an oscillation process canbe used. For purposes of heating, the glass can be exposed to radiantheat and/or to convection heat in a kiln area, whereby temperatures ofabout 700° C. [1292° F.] are normally set in the kiln area. For purposesof toughening, the glass remains in the kiln area until the softeningpoint has been reached. For example, glass having a thickness of about 4mm is normally heated to at least 600° C. [1112° F.] for about 160seconds. After this thermal treatment step, air nozzles arranged atregular intervals in an adjacent segment of a toughening installationblow air uniformly against both sides of the glass. Here, the glass iscooled off to temperatures as low as about 40° C. [104° F.]. Before theheated glass undergoes thermal quenching, it can also be subjected to ashaping process. For example, the heated glass can be bent prior to thequenching so as to obtain curved glass of the type used, for example,for car windshields.

[0022] Glass having the above-mentioned properties can be obtainedespecially advantageously in that a hybrid sol that has beenspecifically adapted to the properties that are to be achieved isadvantageously precipitated onto a conventional soda-lime glass, butalso, for instance, onto a borosilicate glass. The hybrid sol that isaimed at creating the porous antireflection surface coatingadvantageously comprises [SiO_(X)(OH)_(Y)]_(n) particles, wherein 0<y<4and 0<x<2, and whereby the particles comprise a first particle fractionhaving a first particle size range and a second particle fraction havinga second particle size range, and said hybrid sol also contains waterand 2% to 97% by weight of solvent, in a preferred embodiment it cancontain 15% to 30% by weight of solvent, 40% to 70% by weight ofstabilizer and 10% to 35% by weight of water. Therefore, the hybrid solused for the production of the antireflection surface coating comprisesa mixture of large and small SiO₂ particles, thus yielding the twocoating fractions that are tailored for achieving the objectives whenthey are deposited onto the actual glass.

[0023] The hybrid sol can advantageously be obtained by hydrolyticpolycondensation of a tetraalkoxysilane in an aqueous medium containingsolvents, whereby a hydrolysis mixture with silicon oxide-hydroxideparticles having a particle size of 4 nm to 15 nm is obtained, and byadding a monodispersed silicon oxide-hydroxide sol having a meanparticle size of 20 nm to 60 nm and a standard deviation of 20% at themaximum, at a point in time of at least 5 minutes after the addition ofthe tetraalkoxysilane in the aqueous medium containing solvents.

[0024] The hybrid sol can thus essentially be prepared by a suitablecombination of two different sols, whereby however, a simple mixture ofthese sol components is not sufficient to achieve the combinationeffect. In particular, the envisaged effect of the interaction of theparticle fractions is dependent to a great extent on the selection of asuitable point in time for combining the relatively large SiO₂ particleswith the relatively small, reactive SiO₂ particles.

[0025] In order to set properties that are especially favorable and welladapted to the needs, the particle size of the first fraction ofparticles of the hybrid sol is advantageously selected within the rangefrom 4 nm to 15 nm. Advantageously, the second particle size averages 20nm to 60 nm, with a standard deviation of 20%. The weight ratio of thesmall particle fraction to the large particle fraction in the hybrid solis advantageously 25:1 to 1:5, preferably 10:1 to 2:1, especiallypreferably 3:1 to 2:1. The concentration of the SiO₂ particles in thehybrid sol is advantageously between 0.3% and 4% by weight, preferablybetween 1% and 2% by weight.

[0026] Examples of suitable solvents that can be used to prepare thehybrid sol include low aliphatic alcohols such as ethanol or i-propanol,but also ketones, preferably low dialkyl ketones, such as acetone ormethylisobutyl ketone, ethers, preferably low dialkyl ethers such asdiethylether or dibutylether, tetrahydrofuran, amides, esters,especially acetic acid ethylester, dimethyl formamide, amines,especially triethylamine and mixtures thereof.

[0027] In preferred embodiments, alcohols are used as solvents,especially ethanol, methanol, i-propanol, n-propanol. The amount ofsolvent employed is a function of the amount of the silicon compoundsused as the starting material. The concentration of the solvents in thehybrid sol lies between 2% and 97% by weight, preferably at 15% to 30%by weight. As stabilizers in the hybrid sol, glycolethers or ethers ofother alcohols having two or more hydroxy groups in a concentration of10% to 95%, preferably 40% to 70% by weight can be used. Preference isgiven to the use of 1,2-propylene glycol monomethylether.

[0028] The objective of the method for the production of the glass isachieved in that a conventional soda-lime glass is coated with a coatingsolution containing the hybrid sol and subsequently subjected to adrying step. The drying preferably takes place under relatively constantclimate conditions and is preferably carried out in atmospheric air at atemperature of about 20° C. [68° F.] to 25° C. [77° F.], advantageouslyat about 22° C. [71.6° F.], and at a relative humidity of 55% to 65%,advantageously of 60%.

[0029] As was surprisingly found after extensive tests, if theabove-mentioned hybrid sol is used as the basic material for the coatingof the glass and if the above-mentioned parameters are observed duringthe drying step, an antireflection surface coating can be created in theglass that, on the one hand, has at least two particle fractions; on theother hand, the coating produced in this manner, in addition to havinggood optical and good mechanical properties, also exhibits a specialstructural resistance and an especially high adhesion to the glasssubstrate, even without further after-treatment of the type that can benecessary, for example, for the thermal toughening of the glass thatfollows the actual coating.

[0030] The use of the above-mentioned hybrid sol as the startingmaterial for the production of the antireflection surface coatingensures that the surface coating has a particle size distributionpreferably entailing at least two ranges and configured so as to achievethe objective and meet the requirements. As comprehensive examinationshave surprisingly shown, however, it is precisely the subsequent dryingstep, while observing the above-mentioned parameters, that causes thesurface coating to have an especially high mechanical stability and anespecially high-strength adhesion to the glass substrate, even withoutthermal after-treatment measures being necessary for this purpose. Incontrast to the notion that had existed until now, namely, that, inorder to crosslink the silicic acid network and to attain betteradhesion to the substrate, a thermal treatment or temperature exposureis absolutely necessary when applying an antireflection surface coatingon the basis of SiO₂ particles onto a glass substrate, this can now alsobe achieved without a further thermal treatment step.

[0031] This fact is particularly useful when such a coated glass isconfigured as safety glass that has undergone thermal toughening.Especially since there is no longer a need for a subsequent thermaltreatment step in order to sufficiently harden the surface coating, thethermal toughening of the glass for the production of the safety glasscan advantageously be carried out prior to the application of theantireflection surface coating. This especially allows the use of theso-called horizontal toughening technique in a continuous process, forexample, during the production of toughened float glass, without anyproblem. The impressions that the rollers normally leave in the coatingas a result of such a treatment of an already coated glass are thusreliably avoided when the coating process is carried out subsequent tothe thermal toughening process.

[0032] Due to the especially high-quality optical properties, the glassis highly suitable to be used for covering a solar collector or aphotovoltaic cell.

[0033] However, it is especially advantageous to use the glass in awindow element of a greenhouse. This is so because precisely in agreenhouse, the overall transparency of the covering window, that is tosay, the light transmission it can achieve, is of special importance.The reason is that the productivity in the cultivation of plants in agreenhouse depends to a great extent on an adequate supply of daylightso that normally a high optical transparency is particularly desirablein this case. In order to enhance this to a great degree, the coveringwindow of a greenhouse is provided with a window base pane that, in anespecially advantageous embodiment, is configured so as to have anespecially high transparency or light transmission capability. For thispurpose, the window base pane that is in the form of a glass paneadvantageously has an antireflection surface coating of theabove-mentioned type.

[0034] In order to attain an especially favorable light transmissioncapability in this application case, the glass pane advantageously hasan antireflection surface coating with a refractive index of 1.25 to1.40, advantageously 1.25 to 1.38.

[0035] As was also surprisingly found, coating a glass with a coating ofthe above-mentioned type causes the glass to have hydrophilic propertieson its coated side. As a result, when moisture precipitates on such acoated glass, a wetting of the glass surface rather than drop formationtends to occur. Precisely for an application in a greenhouse, this isespecially favorable since, if drops form on roof windows, the plantsunderneath could become wet, which is undesirable.

[0036] Through the use of the glass in a covering window as a skylightin a greenhouse, in case of precipitating moisture, a large surface areaof the window surface becomes wet when moisture precipitates, so thatthe precipitating moisture runs along the window surface and cansubsequently be drained in a controlled manner. This also prevents thelight transmission from being impaired by light scatter resulting fromthe formation of drops. In order to utilize this aspect in an especiallypositive manner, in a particularly advantageous embodiment, theantireflection surface coating is applied onto the side of the glasspane that is intended as the inside of the greenhouse. Thus, the SiO₂coating preferably faces the interior of the greenhouse, so thatprecipitating moisture can be drained in an especially reliable andcontrolled manner. Moreover, the glass pane of the covering window can,of course, also be provided with such a SiO₂ coating on both sides sothat the total achievable radiant transmittance is particularly high.

[0037] Preferably, the covering window is used in a greenhouse, wherebythe greenhouse is equipped with a number of window elements that make upthe roof or side walls, at least one of which elements is configured assuch a covering window.

[0038] The advantages that are achieved with the invention consistespecially in that, through the at least two prevailing particle sizesin the antireflection surface coating in the form of a binary system orbimodal particle size distribution, a special flexibility can beachieved for systematically optimizing the coating with an eye towardsthe various specifications. Through the suitable selection of theparticle sizes, the antireflection surface coating can be specificallyadjusted in such a way as to have especially high-quality opticalproperties as well as particularly favorable mechanical properties,especially in terms of high abrasion resistance. The coated glass isadvantageously used for covering solar energy systems, especially solarcollectors, for car windshields, for windows or building glazing or elseespecially for covering greenhouses. In particular, an abrasionresistance according to DIN EN 1096-2 can be achieved, in which nodamage of the coating could be ascertained, even after 100 cycles with atest weight of 400 grams. The antireflection surface coating also has anespecially homogenous appearance without forming a perceptible streakypattern. The antireflection surface coating can also be used forprism-cut glass or glass that is otherwise structured, while retainingits especially good optical properties.

EMBODIMENT EXAMPLE

[0039] An embodiment of the invention is explained in greater detailwith reference to a drawing. The following is shown:

[0040]FIG. 1—in a schematic view, a glass with an antireflection surfacecoating;

[0041]FIG. 2—in a top view, the coated surface of the glass according toFIG. 1;

[0042]FIG. 3—in a diagram, the particle size distribution of the surfacecoating of the glass according to FIG. 1; and

[0043]FIG. 4—schematically, a greenhouse with a number of windowelements.

[0044] Identical parts are given the same reference numerals in all ofthe figures.

[0045] The glass 1 according to FIG. 1 is intended for use as a coveringglass for a solar collector, for a photovoltaic module or as a coveringwindow in a greenhouse. In order to achieve a particularly highefficiency for the solar collector or for the photovoltaic module and toallow an especially favorable operation of the greenhouse, the glass 1is designed for a particularly high broadband light transmission,whereby efforts are aimed at achieving a relatively high transmissionfor essentially all wavelengths within the solar spectrum. In order tomake this possible, the glass 1 has a porous antireflection surfacecoating 2 on the basis of SiO₂ particles—advantageously on bothsides—which has been applied onto a glass substrate 4. The high radianttransmittance here is achieved by selecting the refractive index of theantireflection surface coating 2 as close as possible to the square rootof the refractive index of glass, that is to say, as close as possibleto n=1.22. Furthermore, the antireflection surface coating 2 isconfigured in such a way that it is particularly well-suited foreveryday use and, moreover, has an especially high abrasion resistance.

[0046] These two criteria and also an optically homogeneous appearanceof the antireflection surface coating 2 are ensured in the embodiment bya specific configuration of the antireflection surface coating 2 interms of the SiO₂ particles of which it consists. In fact, theantireflection surface coating 2 encompasses, in the form of twosubsystems, a combination of a first fraction of SiO₂ particles with asecond fraction of SiO₂ particles, whereby these two fractions differfrom each other in terms of their particle size. Here, the firstfraction comprises SiO₂ particles having a particle size within therange from about 4 nm to 15 nm, whereas the second fraction has SiO₂particles with a mean particle size of about 35 nm, with a standarddeviation of 20% at the maximum. These fractions can be seen in the topview in FIG. 2. As can be seen in FIG. 2, which is a REM image of theglass 1, the antireflection surface coating 2 has essentially thefollowing structures:

[0047] As a first fraction, there is a supramolecular network 6consisting of small SiO₂ particles having a mean particle size of 4 nmto 15 nm. As the second fraction, there are spherical SiO₂ particles 8having a mean particle size of 20 nm to 60 nm embedded into thissupramolecular network 6. The combination of these two fractions resultsin a high abrasion resistance as well as a particularly estheticalappearance of the layer.

[0048] Accordingly, the antireflection surface coating 2 in the presentembodiment exhibits a particle size distribution of the kindschematically shown in the diagram in FIG. 3. The particle sizedistribution has a first particle size range 10 between about 4 nm andabout 15 nm which is occupied by a relatively large number of particles.This particle size range 10, to which, for example, the mean value ofthe occupied interval, in other words, a value of about 10 nm, can beascribed as the first characteristic particle size, is defined byparticles of the first particle fraction.

[0049] In addition, the particle size distribution according to FIG. 3has a second particle size range 12 which is likewise occupied by asignificant number of particles and in which the particle sizedistribution in the embodiment can be described approximately by aGaussian distribution having a standard deviation of about 15%. Themaximum of the Gaussian distribution, that is to say, a value of about35 nm, can be ascribed, for example, to the particle size range 12 asthe second characteristic particle size.

[0050] The systematic combination of the two fractions of SiO₂ particlescomes to the fore particularly well as a result of the particle sizeranges 10, 12. On the basis of the logarithmic depiction of the diagramin FIG. 3, it can also be seen that the number of particles belonging tothe first particle fraction predominates by far over the number ofparticles belonging to the second particle fraction. In the embodiment,the ratio of the number of particles of the first particle fraction tothe number of particles of the second particle fraction is about 500.

[0051] In order to produce the glass 1, the glass substrate 4 is firstcoated with a hybrid sol that is geared specifically towards thepreparation of the surface coating 2, which contains at least twocomponents. The hybrid sol, in turn, is produced in the followingmanner.

[0052] First of all, a tetraalkoxysilane in placed into an aqueousmedium containing solvents, in response to which hydrolyticpolycondensation sets in. The process is carried out essentially asdescribed in German patent DE 196 42 419 under thorough mixing. Ifapplicable, for the hydrolytic polycondensation, a basic catalyst thatshortens the reaction times can be added to this mixture. The use ofammonia is preferred.

[0053] The solvents contained in the hydrolysis mixture can be selectedfrom the above-mentioned solvents. Preference is given to the use ofethanol, methanol, i-propanol or n-propanol, while ethanol is especiallypreferred.

[0054] The hydrolysis takes place at temperatures ranging from 5° C. to90° C. [41° F. to 194° F.], preferably from 10° C. to 30° C. [50° F. to86° F.]. In this context, the tetraalkoxysilane employed yields thesmall silicon oxide-hydroxide particles with a particle size rangingfrom 4 nm to 15 nm.

[0055] After the addition of the tetraalkoxysilane, the hydrolysismixture is thoroughly stirred, for example, by means of agitation, for aperiod of at least 5 minutes.

[0056] Subsequently, a sol consisting of monodispersed siliconoxide-hydroxide particles having a mean particle size of 20 nm to 60 nmand a standard deviation of 20% at the maximum is added to thehydrolysis mixture described above. The time until the siliconoxide-hydroxide sol consisting of monodispersed particles is added tothe hydrolysis mixture depends on the use of condensation catalysts forthe hydrolytic condensation of the silicon compounds. At the earliest 5minutes after the addition of the tetraalkoxysilane to the aqueoushydrolysis mixture containing solvents, the monodispersed siliconoxide-hydroxide sol is added to this mixture. The point in time of thisaddition can be delayed by up to 48 hours after the addition of thetetraalkoxysilane to the hydrolysis mixture. Preferably, this point intime lies between 5 minutes and 24 hours after the beginning of theformation of silicon oxide-hydroxide particles having a particle sizeranging from 4 nm to 15 nm. Special preference is given to time frame of20 to 80 minutes after the start of the reaction.

[0057] If the point in time of the addition is postponed for more than48 hours after the start of the reaction, differences in the propertiesof the hybrid sol can no longer be detected in comparison to an additionwithin 48 hours.

[0058] The point in time of the addition of the silicon oxide-hydroxidesol consisting of monodispersed particles to the hydrolysis mixturedetermines to a decisive extent the properties of the hybrid solutionaccording to the invention. In this manner, a statistical distributionof the monodispersed particles in the small silicon oxide-hydroxideparticles is achieved and an accumulation of the monodispersed particlesin the sense of a “formation of islands” is avoided since the latterwould lead to worse abrasion stability.

[0059] The monodispersed silicon oxide-hydroxide sol is preferably addedto the hydrolysis mixture in one portion.

[0060] In a special embodiment, the silicon oxide-hydroxide solconsisting of monodispersed particles is produced in accordance with themethod described in U.S. Pat. No. 4,775,520. For this purpose, thetetraalkoxysilane is placed into an aqueous-alcoholic-ammoniacalhydrolysis mixture and thoroughly mixed, thus producing primary siliconoxide-hydroxide particles. Any hydrolysable silicic acid orthoesters ofaliphatic alcohols can be employed without problems as suitabletetraalkoxysilanes. First and foremost, the best options here are theesters of aliphatic alcohols having 1 to 5 carbon atoms such as, forinstance, methanol, ethanol, n-propanol or i-propanol as well as theisomeric butanols and pentanols. These can be employed individually aswell as in a mixture. Preference is given to the silicic acidorthoesters of the C₁-C₃ alcohols, especially tetraethoxysilane.Suitable alcohol components are aliphatic C₁-C₅ alcohols, preferablyC₁-C₃ alcohols such as methanol, ethanol and n-propanol or i-propanol.They can be present in the mixture individually as well as in a mixture.The tetraalkoxysilane is preferably added to the mixture in one portion,whereby the reactant can be present in pure form or in a solution in oneof the above-mentioned alcohols. A concentration of tetraalkoxysilane inthe reaction mixture of between about 0.01 mol/l and about 1 mol/l canbe chosen in order to create the primary silicon oxide-hydroxideparticles. Once the reactants have been combined, the reaction sets inimmediately or after a few minutes, which is evident in that thereaction mixture soon becomes opalescent due to the particles that areformed.

[0061] The hydrolysis mixture containing primary silicon oxide-hydroxideparticles is subsequently continuously mixed with additionaltetraalkoxysilane in such a way that essentially no new siliconoxide-hydroxide particles are formed. Rather, the primary siliconoxide-hydroxide particles already present grow to form larger,monodispersed particles.

[0062] Depending on the selection of reactants as well as on theirconcentration in the reaction mixture, particles can be obtained havinga mean particle size between 20 nm and 60 nm and with a standarddeviation of 20% at the maximum.

[0063] It has proven to be advantageous to conduct the reaction toproduce these particles at a high temperature. Favorable temperatures inthis context are those between 35° C. and 80° C. [95° F. and 176° F.],preferably between 40° C. and 70° C. [104° F. and 158° F.]. It turnedout that the particle size scatter decreases at an elevated temperature,although the mean particle size also does. At lower temperatures, thatis to say, at around room temperature, larger particles with a largersize scatter are obtained under otherwise identical conditions.

[0064] A further increase in the stability of the monodispersed siliconoxide-hydroxide sol might necessitate the removal of the alcohol and/orammonia from the sol. This is done on the basis of known techniquesaccording to the state of the art, for example, by raising thetemperature in order to remove the volatile ammonia.

[0065] Here, the term monodispersed refers to particles that exhibit astandard deviation of 20% at the maximum, especially 15% at the maximumand especially preferably 12% at the maximum, and that are essentiallypresent as discrete particles.

[0066] Under thorough mixing, for instance, by means of agitation, thesilicon oxide-hydroxide sol consisting of monodispersed particles isadded to the hydrolysis mixture. At temperatures ranging from 10° C. to40° C. [50° F. to 104° F.], this thorough mixing is continued over aperiod ranging from 1 minute to 48 hours, preferably from 10 minutes to5 hours.

[0067] In the subsequent stage of the process for the production of thehybrid sol, a stabilizer can be added to the hybrid sol. Examples ofstabilizers are glycol ether or ethers of other alcohols. Preference isgiven to the use of 1,2-propylene glycol-1-monomethyl ether.Subsequently, the stabilized sol mixture is thoroughly mixed over aperiod ranging from 1 minute to 24 hours, preferably from 5 minutes to 1hour.

[0068] If necessary, the hybrid sol thus formed can be subsequentlyfiltered. In this case, the filtration through a conventional filter,preferably with a pore size of 1 μm to 5 μm, yields the desired solwhich can then be used for further processing.

[0069] In particular, the hybrid sol can be produced according to thefollowing examples:

Example 1

[0070] 29.4 grams of an aqueous 0.08 n ammonium hydroxide solution arecompletely mixed with 380 grams of ethanol and 50.7 grams oftetramethoxysilane are added while being stirred. After a stirring timeof 150 minutes, 400 grams of a 5%-monodispersed silicon oxide-hydroxidesol containing silicon oxide-hydroxide particles with a mean particlesize of 25 nm are added and stirred for another 60 minutes until 970grams of 1,2-propylene glycol monomethyl ether are added to the batch.The hybrid sol thus produced is subsequently filtered through afiberglass prefilter.

Example 2

[0071] 25.4 grams of polyethylene glycol having a mean molecular weightof 200 g/mol are dissolved in a mixture consisting of 29.4 grams of 0.08n aqueous ammonium hydroxide and 357 grams of ethanol. Then 50.8 gramsof tetramethoxysilane are added to this solution while being stirred.After a stirring time of 125 minutes, 400 grams of a 5%-monodispersedsilicon oxide-hydroxide sol containing silicon oxide-hydroxide particleswith a mean particle size of 25 nm are added and stirred for another 30minutes until 1300 grams of 1,2-propylene glycol monomethyl ether areadded to the batch. The hybrid sol thus produced is subsequentlyfiltered through a fiberglass prefilter.

[0072] The hybrid sol thus obtained is applied onto the glass substrate4 in order to produce the glass 1. For this purpose, the hybrid sol canbe present in a coating solution into which the glass substrate 4 isdipped. Instead of such dip-coating, however, it is also possible toemploy a spraying method or a rotary coating method, also called spincoating.

[0073] For example, as the glass substrate 4, a glass pane that measuresapproximately 1 meter by 1 meter and has a thickness of 4 mm and thathas previously been cleaned with demineralized water and subsequentlydried can be dipped into the coating solution. The glass pane is removedfrom the coating solution at a constant withdrawal speed of 5.5 mm/sec.The glass substrate 4 coated in this manner is subsequently subjected toa drying step in atmospheric air. For this purpose, the coated glasssubstrate 4 is dried at a temperature of about 22° C. [71.6° F.] and ata relative humidity of about 60%. This drying procedure can be done bysimply letting the glass substrate stand exposed to the air or else byblowing air against it. As has been found, an abrasion-resistant surfacecoating 2 having very good optical properties and a special mechanicalstability is already obtained after this drying step and this coatingexhibits approximately the pattern shown in the top view in FIG. 2, inother words, especially a combination of two particle fractions withmean particle sizes that can be clearly differentiated from each other.In particular, there is no need for any additional thermal treatment ofthe thus obtained coated glass 1 in order to achieve sufficientmechanical stability or abrasion resistance.

[0074] The glass 1 is configured as a toughened safety glass. Due to theespecially favorable properties of the surface coating 2, which in factdoes not require any additional thermal treatment after the actualcoating procedure, the thermal toughening is already carried out on theuncoated glass substrate 4. However, the toughening could also beperformed after the coating procedure.

[0075] A commonly employed toughening method is used for the tougheningwhereby this can be a vertical toughening technique, a horizontaltoughening technique in a continuous process or also a horizontaltoughening technique in an oscillation process. In any case, the glasssubstrate 4 is heated up to a temperature of 700° C. [1292° F.] in akiln area, whereby radiant heat and/or convection heat can be employed.Here, the glass substrate 4 remains in the kiln area until the softeningpoint has been reached. The glass substrate 4, having a thickness of 4mm, is heated, for instance, to at least 600° C. [1112° F.] for about160 seconds. Subsequently, the heated glass substrate 4 is quenchedwhereby, for example, air nozzles arranged at regular intervals blow airuniformly against both sides of the glass substrate 4. In this process,the glass substrate 4 is cooled off as low as 40° C. [104° F.].

[0076] During this thermal pre-treatment for toughening purposes, theglass substrate 4 can also undergo a shaping process, for instance, itcan be bent. In the case of glass 1 that has not yet been coated, aftercompletion of the toughening procedure, the glass substrate 4 is thencoated with the surface layer 2 in the manner described.

[0077] The glass 1 produced, thermally toughened and coated in thismanner is particularly well-suited for use as a covering for a solarcollector, for a photovoltaic module or for other optically sensitiveelements although it can also be used in covering windows for agreenhouse 20, as schematically shown in FIG. 4. The greenhouse 20according to FIG. 4 comprises a number of window elements 22 that formthe roof or the side walls and that, in their entirety, form the outerwall of the greenhouse 20; depending on the application purpose, theycan be configured as fixed or tilting elements. For purposes ofmechanical stabilization, the window elements 22 are held by a supportframe 24; as an alternative, however, the window elements 22 can also beconfigured in form of a self-supporting structure that dispenses with atubular frame of its own.

[0078] Different requirements can be made of the transparency of theindividual components when it comes to the enveloping or outer surfacearea of the window elements 22 that form the greenhouse 20, and the sameapplies to their corresponding use in other buildings or in technicaldevices such as, for example, solar collectors. For instance, dependingon the time of day or the season of the year, these requirements canvary. Moreover, especially in the case of covering window materials forgreenhouses and solar collectors, the highest possible transparency isusually expected. This is why the window elements 22 are designed tohave an overall particularly high transparency that allows an especiallyhigh yield of daylight inside the greenhouse 20 and thus a particularlylow product-specific energy consumption for the cultivation of plants.

[0079] For this purpose, the or each window element 22 is provided witha glass plate as the window base pane which is configured as anantireflection-coated glass 1. The window base pane here is configuredas a thermoset safety glass. Here, the SiO₂ coatings have a refractiveindex of about 1.25 that allows a particularly high level oftransparency to be set.

List of Reference Numerals

[0080]1 glass

[0081]2 antireflection surface coating

[0082]4 glass substrate

[0083]6 network

[0084]8 particle

[0085]10,12 particle size range

[0086]20 greenhouse

[0087]22 window element

[0088]24 support frame

1-17. (canceled)
 18. A coated glass product comprising: a glasssubstrate; and a porous antireflection surface coating includingSiO₂-based particles having a first particle fraction including a firstcharacteristic particle size range and a second particle fraction havinga second characteristic particle size range that is different from thefirst characteristic particle size range.
 19. The coated glass productas recited in claim 18, wherein the first particle size range is 4 nm to15 nm.
 20. The coated glass product as recited in claim 18, wherein thesecond characteristic particle size range is defined by a mean particlesize range of 20 nm to 60 nm, and a standard deviation of 20% at themaximum.
 21. The coated glass product as recited in claim 18, whereinthe first fraction includes a first number of particles and the secondfraction includes a second number of particles, and wherein a ratio ofthe first number of particles to the second number of particles is3000:1 to 100:1.
 22. The coated glass product as recited in claim 21,wherein the ratio is 1000:1 to 250:1.
 23. The coated glass product asrecited in claim 18, wherein the coated glass product is configured as atoughened safety glass.
 24. The coated glass product as recited in claim18, wherein the surface coating includes a hybrid sol comprising 2% to50% by weight of water, 2% to 97% by weight of solvent, and 0.3% to 4%by weight of [SiO_(X)(OH)_(Y)]_(n) particles, wherein 0<y<4 and 0<x<2,and wherein the glass substrate includes conventional soda-lime glass.25. The coated glass product as recited in claim 24, wherein which thehybrid sol is obtainable from a hydrolytic polycondensation of atetraalkoxysilane in an aqueous medium containing solvents.
 26. Thecoated glass product as recited in claim 24, wherein the firstcharacteristic particle size is 4 nm to 15 nm.
 27. The coated glassproduct as recited in claim 24, wherein the second particle fraction hasa mean particle size of 20 nm to 60 nm.
 28. A method for producing acoated glass product, comprising: providing a soda-lime glass substrate;preparing a coating including a hybrid sol comprising 2% to 50% byweight of water, 2% to 97% by weight of solvent, and 0.3% to 4% byweight of [SiO_(X)(OH)_(Y)]_(n) particles, wherein 0<y<4 and 0<x<2, andwherein the particles comprise a first particle fraction having a firstcharacteristic particle size and a second particle fraction having asecond characteristic particle size, and coating a surface of the glasssubstrate with the coating.
 29. The method as recited in claim 28,wherein the preparing the coating includes performing a hydrolyticpolycondensation of a tetraalkoxysilane in an aqueous medium containingsolvents so as to obtain the hybrid sol.
 30. The method as recited inclaim 28, wherein the hydrolytic polycondensation includes the steps of:obtaining a hydrolysis mixture with silicon oxide-hydroxide particleshaving a particle size of 4 nm to 15 nm; adding the tetraalkoxysilane inthe aqueous medium containing solvents; and at least five minutes later,adding a monodispersed silicon oxide-hydroxide sol having a meanparticle size of 20 nm to 60 nm and a standard deviation of 20% at themaximum.
 31. The method as recited in claim 28, further comprising,after the coating step, drying the coated glass in atmospheric air at atemperature of about 20° C. to 25° C. and at a relative humidity of 55%to 65%.
 32. The coated glass product as recited in claim 28, furthercomprising, after the coating step, drying the coated glass inatmospheric air at a temperature of about 22° C. and at a relativehumidity of 60%.
 33. The method as recited in claim 28, wherein furthercomprising thermally toughening the glass substrate prior to the coatingstep.
 34. The coated glass product as recited in claim 1, wherein thecoated glass product is part of at least one of a solar collector, aphotovoltaic cell, and a greenhouse.
 35. The coated glass product asrecited in claim 34, wherein the antireflection surface coating has arefractive index of 1.25 to 1.40.
 36. The coated glass product asrecited in claim 34, wherein the antireflection surface coating has arefractive index of 1.25 to 1.38.
 37. The coated glass product asrecited in claim 34, wherein the coated glass product is part of agreenhouse and the antireflection surface coating is disposed on asurface of the glass substrate facing an inside of the greenhouse.